1. Field
This invention relates to the field of digital image processing, particularly the operation of CMOS active pixel sensors for increased sensitivity.
2. Related Art
CMOS active pixel sensors have become a viable alternative to CCD imagers for many digital imaging applications such as digital movie and still cameras, camera cell phones and security cameras. The CMOS imager technology is advantageous for many reasons including, but not limited, low power requirements and low cost and ease of manufacture based on the fact that CMOS imagers can be produced on almost any standard CMOS fabrication line. However, current CMOS image sensors do have drawbacks that effect quality and performance. In particular, the sensitivity of CMOS active pixel sensors can be negatively impacted by the typical configuration and operation of the pixels.
A typical CMOS active pixel sensor, as depicted schematically in FIG. 1a, comprises of a photo-detector, 102, a RESET transistor, 104, a source follower transistor, 106, an access transistor, 108, and optionally an additional transfer gate. When photons strike a CMOS active pixel sensor electron-hole pairs are generated in the silicon. The number of charge carriers generated is directly proportional to the level of the incident light. The bulk of the charge carriers then diffuse to the photo-detector. However, there are a significant number of charge carriers that diffuse to other sites. The charge carriers collected by the photo-detector can then be read out for each pixel in a sensor array and a composite of the light level detected at each pixel can be used to represent an image. Charge carriers that diffuse to other sites are lost and thus have the effect of lowering the overall sensitivity of the sensor. Among these other sites are the reverse biased, highly doped N type diffusions of the in-pixel source/drain of the transistors. Electrons that are lost the in-pixel transistors not only represent the loss of signal affecting sensitivity, but they can also affect the end image quality via leakage and noise they can cause.
To improve CMOS imager sensitivity in certain lighting and imaging situations, there is a need to improve loss of CMOS active pixel sensor sensitivity due to the diffusion of charge carriers away from the in pixel photo-detectors.
Typically, in the structure and operation of CMOS active pixel sensors, the source or drain of the RESET transistor, 104, and the source follower transistor, otherwise known as the reference line in the art, are held at some non-ground voltage, Vdd, 110. During the charge collection, or integration, period both transistors' sources and drains are sitting at Vdd. This voltage causes a non-zero electric field at the P-N junction that can attract the generated charge carriers.